1. Field of the Invention
This invention relates to a semiconductor relay, and more specifically to a semiconductor relay for transmitting high frequency signals.
2. Description of the Prior Art
FIG. 1 shows the structure of a prior art semiconductor relay. As shown, this semiconductor relay has a light emitting diode (referred to as LED, below) 101 in its primary side. The secondary side of this relay is comprised of a photo diode array 102 for generating electromotive force on the reception of light emitted by LED 101, a resistor 103, a diode array 104 parallel-connected with resistor 103, an p-channel junction FET (referred to as J-FET, below) 105, and one pair of MOSFETs 106a and 106b series-connected oppositely to each other. Diode array 104 and J-FET 105 form a circuit for discharging the gates of MOSFETs 106a and 106b.
According to the above mentioned structure, photo diode array 102 does not generate a voltage unless LED 101 emits light. In this case, J-FET 105 is held "on" so that the respective substrates and gates of MOSFETs 106a and 106b conduct. This makes both MOSFETs 106 to be "off".
On the contrary, once a current flows through LED 101 and it emits light, photo diode array 102 generates an electromotive force in a forward direction. This makes J-FET 105 to turn off and a positive voltage is applied to the respective gates of MOSFETs 106a and 106b. As a result, both MOSFETs 106a and 106b turn on to connect output terminal 108a with output terminal 108b.
The prior art semiconductor relay mentioned above has the following disadvantage. As far as the relay is in an "off" state, terminals 108a and 108b of MOSFETs 106a and 106b are separated to each other. In this case, a certain capacitance still exists between terminals 108a and 108b. Due to this capacitance, the impedance between terminals 108a and 108b lowers especially for high frequency signals. As a result, terminals 108a and 108b can't be separated completely. This impedance can be calculated from the following equation. EQU Z=1/(2.pi.Cf) (1)
where C means the capacitance between terminals 108a and 108b and f means the frequency of an input signal.
Assume that C=100 pF and f=10 MHz. In this case, the impedance is 159.OMEGA., which is too small to say that this relay is in a complete "off" state. In order to obtain a sufficiently high impedance, such as 1.59.OMEGA., C must be reduced to about 1 pF.
As shown in FIG. 2, the total capacitance developed across output terminals 108a and 108b is composed of capacitance Cpin, which exists between the package terminals, and junction capacitance 2Cj of MOSFETs 106a and 106b. In usual, Cpin is less than 1 pF while Cj is between scores to hundreds pF. Consequently, the capacitance developed across output terminals depends mainly on the junction capacitance.
Junction capacitance Cj of each MOSFET 106a or 106b arises out of the depletion layer and it is expressed in the following equation. EQU Cj=.epsilon.a/l (2)
where .epsilon. means the dielectric constant of the semiconductor material, a means the junction area, and l means the width of the depletion layer. As is clearly understood from equation (2), depletion layer width l and junction area a should be reduced in order to reduce Cj. The on-resistance of a MOSFET, however, increases as width l and junction area a reduce. This is understood from the following equation. EQU Ron=.rho.l/a (3)
where .rho. means the resistivity of the semiconductor material.
On the other hand, the width of the depletion layer determines the withstand voltage of a MOSFET. Accordingly, the area of the junction determines the resistance of the MOSFET, as far as the same kind of semiconductor material is used. FIG. 3 is a graph showing the correlation among the capacitance of the output terminals, the on-resistance, and the chip size of MOSFETs. As is clearly understood from the above mentioned explanation, the on-resistance increases as the chip size decreases. On the contrary, the capacitance of the output terminals increases as the chip size increases. In usual, the product of an output terminal capacitance and an on-resistance is used as the performance index of a semiconductor relay.
FIG. 4 is a graph showing the correlation between the product mentioned above and the chip size. As is easily understood from this-figure, the total performance of the prior art relay has not been improved even if the junction area has been changed. In this case, the chip size is almost proportional to the junction area.
As mentioned above, two basic requirements for semiconductor relays, which are to reduce the on-resistance and to increase the off-resistance of this relay, oppose each other due to the technical reason.